This invention relates generally to displacement detection apparatus, and more particularly, is directed to an optical triangulation displacement sensor.
In conventional optical triangulation displacement sensors, a radiant or infrared ray, such as laser light, is projected on an object in order to measure the distance thereof from a point of reference. A portion of the light is scattered from the surface of the object and is imaged by a converging lens on a detector including a plurality of light sensitive elements, such as photodetectors, arranged at fixed distances from the object. If the surface of the object is displaced so that the light spot is shifted along the path of the laser beam, the image of the light spot on the detector is also shifted. The distance to the object is measured by determining which light sensitive element receives the most light reflected from the object. Examples of such devices are described in U.S. Pat. Nos. 4,274,735 to Tamura et al, 4,522,492 to Masunaga, and 4,733,969 to Case et al. See also, U.S. Pat. Nos. 4,660,970 to Ferrano, 4,873,449 to Paramythioti et al and 4,897,536 to Miyoshi.
It is known that the relation between displacement of the light spot on the object surface and the displacement of the image of the light spot on the detector array is non-linear. However, prior art optical displacement sensors have generally used a linear approximation of the non-linear relation. Methods for displacement measurement that recognize the existence of a non-linear relationship have also been proposed. In U.S. Pat. No. 4,705,395 to Hageniers, a reference surface on an encoded table is used to calibrate the output of a triangulation sensor measuring distance. The table is moved through several steps and the various points are used to obtain a calibration curve via a least squares fit, this being the best choice for a third-order polynomial. Once the sensor has been taken through the calibration and the appropriate polynomial coefficients have been determined, then the system is structured such that the photodiode array reading is used to operate in a look-up table fashion for readout. Then, when the sensor is used, and an actual reading is required, the diode array reading is used as an address in the look-up table to provide a quick answer for the desired measurement.
However, not only is the polynomial an approximation, so that the exact non-linear relation is not known from this patent, but also, this patent provides a fitted polynomial based on test data. In other words, it is based only on experimental data. This means that, for each situation, experimental data must be provided, which is extremely burdensome. In addition, a look-up table must then be provided, which occupies memory space.
See also U.S. Pat. Nos. 4,864,147 and 4,761,546, both to Ikari et al, which disclose linearity correcting means to provide a mathematical correction number to correct any non-linearity of the measured distance signal, in order to provide linearity to the measured distance signal.
With these latter patents, the non-linear relation which is linearized, is provided only for the situation where the detector array is perpendicular to the optical axis of the receiving lens. As a result, there is no consideration of the angle .phi. between the optical axis of the receiving lens and the orientation of the photodetector. However, if the angle .phi. changes, as is common, the non-linear relation of this patent does not apply.
In this regard, this angle .phi. will generally be other than 90 degrees. This is because, with the angle .phi. equal to 90 degrees, light reflected from the object and through the receiving lens, will not focus properly on the position detector. In other words, to maintain the condensed light in a good focused condition, the photodetector has to be oriented at an angle .phi. to the optical axis of the receiving lens. The value of the angle .phi. depends on the design parameters of the triangulation device and is given by the formula: tan .phi.=(d.sub.o /d.sub.i) tan .theta..
U.S. Pat. No. 4,655,586 to Stauffer discloses an adjustable zone proximity sensor utilizing two pairs of detectors, at least one of which is adjustable in position so as to produce unique signals when an object is in a near zone, medium zone and far zone. However, adjustment of the detectors is made in accordance with the distance from the object, and not on the basis of a non-linearity correction.
Other devices of related interest, but of less importance, are those described in U.S. Pat. Nos. 3,661,465 to Groh, 4,004,852 to Pentecost, 4,368,383 to Shirasu et al, 4,900,146 to Penney et al, and 4,902,130 to Bouillot et al.